1. Field of the Invention
The invention relates to cellular communication systems, and particularly to the optimization of radio coverage and capacity at a cell level. The invention is particularly but not exclusively concerned with time division multiple access (TDMA) systems.
2. Description of the Related Art
A typical cellular radio network may include of a system in which radio cells have both fixed coverage and fixed capacity performance, which can lead to extensive downlink capacity overhead and wasted resources. A time division multiple access (TDMA) system is an example of such a system.
Such systems are either multicarriers or single carrier systems. A multicarrier system includes a single transmission unit that can broadcast a plurality of carriers. A single carrier system includes a single transmission unit that can broadcast a single carrier. In such single carrier systems, in order to increase capacity, it is necessary to use multiple transmission units, and combine the multiple carrier signals to an antenna.
Typically, a multicarrier system has a certain maximum downlink transmit power that is divided between the carriers used in the cell. That is, the more carriers in use means that less downlink power is allocated to the carriers (users), leading to diminished coverage of the radio cell. In a single carrier system, the addition of new carriers (to increase capacity) leads to increased combining losses if the amount of antennas is not increased. Without proper control of these systems, capacity/coverage overhead exists, further leading to wasted resources. Network planning may also become a problem.
In general, existing GSM radio networks are planned so that the cell coverage is determined by the carrier for the broadcast control channel (BCCH). All other traffic carriers, if present, have the same coverage as the BCCH, with the same service. This kind of a system provides the same service for the same amount of users over the whole cell area, and does not take into account a user's location in the cell area. Typically this is a waste of resources.
A particular problem is related to the downlink in multicarrier systems. In a multicarrier system, the power that is allocated to the broadcast control channel determines the coverage of the entire cell, and the power which is ‘left over’ is used for other traffic carriers. If the output power of the system is 40W and the operator decides that the broadcast control channel is to be transmitted with 36W (i.e. 90% of the total power), then this leaves only 4W available for the remaining traffic channels. If, for example, there are four further traffic channels, then the use of such traffic channels with the available power is clearly inefficient, as such channels are effectively useless.
As such, there is a need for network operators to carefully plan network design in the case of multicarrier systems.
Two techniques have attempted to provide improved solutions in single carrier systems.
One such technique is a first single carrier system which is known as intelligent coverage enhancement (ICE). This is a technique where the same area or sector is covered by two cells with different output powers, and these two cells share common antennas. Because of the difference in the power between the two cells, the coverage areas are not entirely coincident. However it is characteristic of this arrangement that the coverage area of the cell having a smaller output power is totally within the coverage area of the cell having the higher output power. This allows the use of common antennas.
An enhanced arrangement known as ICE+ allows different kinds of configuration, including different numbers of transmitters and antennas. However for both ICE and ICE+ functionality the coverage and capacity of the cells is fixed by the base transceiver station hardware.
The other technique is a second single carrier system which is known as intelligent underlay/overlay (IUO). IUO is a feature designed to allow an operator to reuse frequencies more intensively, and hence achieve a higher radio network capacity. In order to achieve a higher radio network capacity by means of IUO, the operating spectrum of the network is divided into regular frequencies and super-reuse frequencies. Continuous coverage areas provided by the overlay network utilize irregular frequencies. The super-reuse frequencies are intended to serve mobile stations which are, for example, close to the base transceiver station.
However, in IUO the coverage and capacity is again fixed by the base transceiver station hardware.